[xquery-talk] XML Databases that support "Dynamic" XQuery Collections?

[Wood, Jamey]

Are there any XML Databases (or even just XQuery engines that use some kind of persistent optimized indexing scheme) that provide some way to define "dynamic" collections?  In other words, I'm looking for something similar to how Saxon implements collections:

  http://www.saxonica.com/documentation/sourcedocs/collections.xml

…where:

  * Any given document could belong to multiple different collections simultaneously
  * A collection's membership might itself be produced by a query or other dynamic mechanism

Sorry if that isn't the right terminology for what I'm seeking.  From what I've read and seen, Saxon's approach to collections could meet these needs.  But its general performance will be hindered by not having some kind of persistent indexing mechanism for the underlying XML documents (as is done in systems like eXist and MonetDB).  Is that correct?

Thanks,
Jamey